1. Field of the Invention
The present invention relates to a radio receiver, and more particularly to improvements of the signal-to-noise ratio of an audio output of an on-vehicle radio receiver.
2. Prior Art
Generally speaking, when the input to a receiver through a receiving antenna decreases, the noise in the receiver output increases deteriorating the signal-to-noise ratio. Japanese Utility Model Publication No. 59-6015 in FIG. 14 discloses a receiver having two filters in the IF stage; a wide filter and a narrow filter. The outputs of these two filters are selectively supplied to an FM demodulator such that the IF signal from the wide filter is normally demodulated while the IF signal from the narrow filter is demodulated when the signal strength to the receiver decreases. This type of conventional on-vehicle receiver may suffer from the problem that when the signal strength of an adjacent station becomes strong compared to that of the station presently being received while the vehicle is running, the signal of the adjacent station interferes with the signal currently being received.
Even if the desired station is not interfered with by an adjacent station, the frequent changes in field strength of the station currently being received causes a frequent switching operation between the narrow filter and wide filter, thereby causing producing poor quality of sound.
If the narrow and wide filters do not have the same frequency characteristics, pulse noise such as an ignition noise cannot be removed. In which case, the receiver suffers from the problem that the signal output is frequently switched between the wide filter and narrow filter.
If the signal currently being received is overmodulated or multipath noise exists, the difference in gain and bandwidth between the wide filter and narrow filter results in the difference in output between the two filters, which leads to the malfunction in switching between the two filters.
A large amount of multipath noise may cause an interfering signal to become stronger than the signal currently being received. Then, the strongly interfering signal may actually interfere the signal currently being received. Thus, the D/U ratio varies very frequently causing a frequent switching operation between the wide filter and narrow filter.
When in the seek operation, where stations in the frequency band are scanned to automatically select a desired station, the strength of incoming-signals are often weak resulting in a difference in signal level between the wide and narrow filters. If the receiver is designed to operate such that IF signals passing through the two filters are compared, the receiver determines there is interference if the difference between the two IF signals is above a predetermined level. A difference above a predetermined level that occurs during the seek operation, is interpreted as being due to interference. Thus, the narrow filter is accidentally selected rather than the wide filter. Furthermore, the detected outputs of the two IF signal paths are unstable and different in level for a short time after seek operation is over. Thus, there is the problem that the narrow filter is selected without regard to whether interference exists.
The difference in gain between the wide and narrow filters causes the degree of muting the audio signal when the two filters are switched from one to another, resulting in uncomfortable listening condition. Conventionally, the muting circuit and detector are fabricated in different integrated circuits, leading to complex circuit configuration and high manufacturing costs.
When the receiver incoming signal decreases and the receiver is interfered by an adjacent station, the S-meter is deflected by the interfering signal. Thus, automatic selection of stations by the aid of the RDS system is not performed properly leaving the receiver output interfered.
In an RDS receiver shown in FIG. 15, the frequencies of network stations are stored in the RDS decoder in addition to the station currently being received. When the signal strength of the station currently being received decreases, the S-meter signal level also decreases. The decreased S-meter signal is supplied to the RDS decoder which in turn outputs the frequency of an alternative network-station to the PLL. If the new station is also weak in signal level, then the decoder outputs the frequency of another alternative station. In this manner, network stations are searched until a station of reasonably strong signal level is received. If the receiver receives interference by the adjacent station due to the decrease of signal strength of the station currently being received, the S-meter is deflected by the interfering signal, failing to properly detect the decrease in level of the desired signal. Thus, the interference with the receiver remains.